Water jet powered watercraft have become very popular in recent years for recreational use and for use as transportation in coastal communities. Water jet propelled watercraft offer high performance, improved acceleration and handling, and shallow-water operation. Accordingly, personal watercrafts (PWCs), which typically employ water jet propulsion units, have become popular, especially in resort areas. As the use of PWCs has increased, a desire for improved performance, including greater operational efficiency, also has increased.
Typically, water jet powered watercraft, such as PWCs, have a water jet propulsion system mounted within the hull that ingests water from a body of water and expels the water at a high velocity from the stern to propel the watercraft. For directional control, a nozzle is generally provided at the outlet of the jet pump and turning is achieved by redirecting the flow of water from the nozzle.
In the typical arrangement for a water jet propulsion unit, an engine output shaft is rotationally coupled to a drive shaft. The drive shaft extends into a water passage, which is defined in part by the hull of the watercraft partially below the water line. The water passage extends from a point forward of the rear of the watercraft to the rear of the watercraft. An impeller disposed within a pump housing portion of the water passage is attached to the drive shaft.
FIG. 12 shows a prior art water jet propulsion system 600 disposed within a hull 612, of which only a portion is shown in broken lines. As shown, an inlet grate 642 is disposed at an inlet 686 to an intake ramp 688. The inlet grate 642 prevents large rocks, weeds, and other debris from entering the water intake ramp 688 and passing through the water jet propulsion system 600. A pump support 650 or ride shoe forms the bottom portion 692 of the water intake ramp 688. The pump support 650 is coupled to the hull 612 within a tunnel 694 through fasteners and/or adhesives (not shown). The pump support 650 includes a main body portion 651 having a vertical attachment surface 652, a forward attachment location 654 that is secured to a ride plate 696, and a ramp portion 656. The ramp portion 656 forms the bottom portion 692 of the water intake ramp 688.
From the water intake ramp 688, water enters into a jet pump 660. The jet pump 660 includes an impeller 670 and a stator 680. The impeller includes blades 672 that extend from a center portion 674 that is coupled to an engine by one or more shafts 698, such as a drive shaft and/or an impeller shaft. The rotation of the impeller 670 pressurizes the water, which then moves over the stator 680 that comprises a plurality of fixed stator blades 682. The role of the stator blades 682 is to decrease the rotational motion of the water so that almost all the energy given to the water is used for thrust, as opposed to swirling the water. As shown, the impeller 670 and the stator 680 are both disposed within a jet propulsion unit housing 690 or pump housing. However, it is also known to position the stator 680 at a position outside of the housing 690 at a position downstream of the housing 690. The housing 690 includes a peripheral wall 691 which defines a passage through which water passes. A forward end 692 of the housing peripheral wall 691 is attached to the vertical attachment surface 654 or the pump support 650. The forward end 692 of the housing peripheral wall 691 defines the inlet into the housing 690.
Once the water leaves the jet pump 660, it goes through a venturi 610. In this prior art water jet propulsion unit 600, the venturi 610 is disposed at the rearward end of the housing 690. Since the venturi's exit diameter is smaller than its entrance diameter, the water is accelerated further, thereby providing more thrust. As shown, the venturi 610 is integrated into the housing 690 and comprises the outlet from the housing 690.
A steering nozzle 602 is pivotally attached to the venturi 610 so as to pivot about a vertical axis 604. The steering nozzle 602 is operatively connected to a steering mechanism such as a steering handlebar (see, e.g., the steering handlebar 74 shown in FIG. 1). Rotation of the steering handlebar causes the steering nozzle 602 to pivot around the vertical axis 604, thereby directing the water discharge to result in a change in the steering direction of the watercraft.
A water passage 695, through which water passes from left to right, is illustrated in FIG. 12. Moving from left to right in this illustration, which is upstream to downstream, the water passage 695 is defined by the inlet 686, the water intake ramp 688, the pump support passage 653, the jet pump 660, the venturi 610 and the steering nozzle 602.
When the amount of water passing through the water jet propulsion system 600 is not optimized, it is possible that cavitation may occur as a result of operation of the impeller 670. Cavitation occurs when an object, such as the impeller 670, moves through a fluid, such as water, to cause turbulence and, at a sufficient speed, creates pockets of vapor. In other words, the impeller 670 can rotate so quickly that, at the tips of the impeller blades 672, a sufficiently low pressure region may be created that the water will flash into vapor, creating small vapor bubbles. When the vapor bubbles collapse, the shock of the collapse can degrade the impeller blades 672 (especially at the tips of the blades 672) by “eating away” at or pitting the blades 672. In addition, cavitation also has the undesired effect of producing noise and vibration that also degrade the operational efficiency of the water jet propulsion system 608. In addition, noise and vibration increases the stress and wear and tear on the impeller 670 and components attached thereto.
In addition, when the watercraft is accelerating from a stand still or a low speed condition, the water drawn through the inlet 686 by the action of the pump 660 experiences a drop of static pressure, which is a condition that promotes cavitation. This undesirable drop of pressure can be minimized by increasing the size of the inlet 686, thus optimizing the system for the acceleration mode. In order to increase the flow of water drawn through the inlet 686, vanes or fins are placed in the vicinity of the inlet 686 and well known to those skilled in the art as a “top loader” (not shown) are also commonly used. U.S. Pat. No. 5,114,368 teaches a water jet propulsion system having such a top loader.
Conversely, as the speed of the craft increases, the static pressure in the inlet builds up which leads to a condition that minimizes the formation of cavitation bubbles in the flow, thus improving the propulsive efficiency of the pump 660. However, as the craft's speed increases, the volume of water forced through the inlet 686 increase and reaches a level where it is greater than the volume of water pulled by the jet pump 660. When the watercraft is traveling at high speed, such increasingly high pressure in the area of the inlet 686 and intake ramp 688 may eventually result in the stern of the watercraft to be pressured up and eventually, the bow of the watercraft to dip in the water, which may, in certain circumstances, cause sudden loss of speed and control of the watercraft. Such phenomenon may occur at various speeds depending on the particular design of each watercraft, the size of the opening of the inlet 686 and operation conditions. A larger inlet opening designed to improve the watercraft's acceleration performances will exacerbate this problem and will result in loss of speed and control at lower traveling speed compared to a watercraft having a smaller inlet.
On the other hand, since a large inlet 686 cuts into the planning area of the hull thus increasing the drag, an inlet 686 optimized for acceleration from low speed will also yield lower propulsive efficiency at high speed. Conversely an inlet 686 optimized for high speed will result in poor acceleration performance due to the occurrence of cavitation.
In view of the foregoing, a need has developed for a watercraft with a water jet propulsion system that provides improved operational efficiency. In order to address this need, water jet propulsions systems with variable inlet sizes have been developed.
U.S. Pat. No. 6,872,105 teaches a water jet propulsion system having a mobile structure disposed within the water passage at a position upstream of the jet pump that modulate the amount of water that is allowed to pass through the water passage. The structure can be a flexible fluid filled bag, an adjustable ride plate, and an additional water passage. According to this patent, each of those structures allows a greater amount of water into the water passage during acceleration than when the watercraft travels at higher constant speed.
U.S. Pat. No. 5,658,176 teaches a water jet propulsion system having water passage with an adjustable inlet which adjusts in size according to the traveling speed of the watercraft. The particular system disclosed comprises fixed and floating vanes pivotally attached along their leading edges to longitudinal structures of the inlet of the water passage so that adjustable inlet openings are created between adjacent floating vanes. According to U.S. Pat. No. 5,658,176, as the speed of the watercraft increases, the inlet openings are closed by the floatable vanes and therefore the volume of water forced through the intake ramp is reduced.
A drawback of the systems taught by these patents is that they comprise numerous movable parts that present risks of breaking and premature wear.
Therefore, there is still a need for a watercraft with a water jet propulsion system that provides improved operational efficiency both during acceleration and at high traveling speed without increasing risks of breaking and premature wear.